This invention relates generally to the field of printed wiring board manufacture. In particular, this invention relates to the manufacture of printed wiring boards having non-plated through holes.
The manufacturing process for a printed wiring board typically involves the formation and plating of through holes in a substrate. In a typical process, these plated through holes are formed by a sequence of steps including drilling, electroless metal plating such as electroless copper plating, resist application, electroplating of copper and a metal resist such as tin or tin-lead, resist stripping, copper etching and metal resist stripping.
The majority of holes in a printed wiring board substrate are designed to act as conductors for the passage of current from one side of the board to the other and are therefore to be plated with a conductive metal. However, a small number of through holes are instead designed for mechanical purposes such as for the attachment of devices to the finished board or attachment of the finished board to a sub-assembly. In such cases, the through holes must often conform to strict dimensional tolerances which may be difficult to maintain if the hole is plated. Designers of printed wiring boards often choose to have such holes be free of plated metal and to achieve the dimensionality target by drilling the hole at a desired diameter.
During electroless copper plating of the printed wiring board substrate, an electroless copper plating catalyst such as palladium colloid is first applied to the substrate. Such catalyzed substrate is then subjected to an electroless copper plating bath. A thin copper layer is then electrolessly plated on all surfaces of the substrate exposed to the plating catalyst, including the through holes designed not to be plated. In a pattern plating process of manufacturing printed wiring boards, a resist, such as a dry film plating resist, is then applied to the substrate in such a way that such holes are covered by the resist film. With dry film resists, such process is often referred to as “tenting.” The dry film resist is effective in preventing copper from being deposited into such holes during subsequent electrolytic deposition of copper. When such resist is later removed, the thin electroless copper deposit in the holes is readily removed by a subsequent etching step. In a panel plating manufacturing process, a thick copper layer is electrolytically deposited over the entire surface of the substrate. A resist layer is then applied and imaged. Copper on areas that are not designed to have a copper trace, including certain through holes, is then removed by etching.
Once a printed wiring board has reached the stage in the manufacturing process where the through holes designed to be plated have been metal plated and the surface patterns created (i.e. a circuitized board or substrate), a selective permanent resist (i.e. a solder mask) is typically applied. A soldermask leaves open areas of the printed wiring board to which electrical components will be attached, such as by soldering or wirebonding. Additional coatings are often applied to the exposed copper features to facilitate subsequent assembly steps. One such coating is nickel or nickel-gold. Nickel-gold coatings typically consist of a layer of electroless nickel followed by a thin layer of gold which is typically immersion plated onto the nickel layer. Such nickel-gold layer possesses excellent solderability and shelf life characteristics. Such electroless nickel-immersion gold is often referred to as “ENIG.”
While such ENIG process is designed to plate solely on the exposed copper features of the printed wiring board substrate, a common problem encountered is that electroless nickel plating may begin in the through holes that are designed not to be plated. While not intending to be bound by theory, it is believed that such electroless nickel plating results from traces of electroless copper plating catalyst, such as palladium colloidal catalyst, left on the hole wall or on traces of copper that the etching process failed to completely remove from the barrel of the hole. Electroless metal plating baths other than nickel can also suffer from metal deposition in the through holes that are not designed to be plated. The formation of such undesired electroless metal deposit, which either partly or completely covers the walls of the holes designed to remain “unplated”, is undesirable both functionally and cosmetically.
In an attempt to prevent the electroless nickel plating of such holes, conventional printed wiring board manufacturing methods include the sequential steps of contacting the printed wiring board substrate with a cleaner, a hole conditioner and a microetch prior to the electroless metal deposition process steps. Cleaners are typically acid or alkaline baths containing surfactants and optionally chelating agents. Such cleaners remove organic contaminants such as grease, oil and fingerprints as well as copper oxidation and debris in the through holes. Hole conditioners are typically sulfur containing organic compounds such as thiosulfate or thiourea which are used to “poison” any electroless catalysts remaining in through holes that are not to be plated. However, such sequential process is not completely effective in preventing the electroless metal plating, and particularly electroless nickel plating, of such through holes and may also have adverse effects on subsequent ENIG plating steps.
When conditioners containing alternatives to thiourea or thiosulfate are used, or when processes that avoid such conditioners altogether are used, the resulting ENIG deposit is often uneven and has a matte finish. Neither of these characteristics is desirable.
There is a need for a process of manufacturing a printed wiring board such that through holes that are designed to remain unplated are not plated with electroless nickel. There is also a need for a process of manufacturing a printed wiring board where final finishes, such as ENIG, are even and bright, particularly when processes free of thiourea or thiosulfate or containing reduced amounts of thiourea are used.